There have been used a various kind of pipe joints such as flange joints, screw type pipe joints and butt weld type pipe joints. In applications where corrosion resistance is of paramount importance, pipe joints made of stainless steel have been used. When these pipe joints are used to connect pipes, they generally need a connecting process such as mechanical fastening of welding and an accuracy of the connecting process directly affects whether leakage occurs or not. Therefore, not only it is important to perform discreetly the process but also the working by itself is complicate and requires well trained workers. When the working is conducted to an existing pipe arrangement, the working such as mechanical fastening or welding may be impossible because of a possibly narrow working space depending upon the particular place where connection of pipes is required.
Also proposed are some pipe joints made of shape memory alloys such as Ni-Ti alloys and Cu alloys. These pipe joints are to make use of shape recovery function by change in temperature of shape memory alloys to connect pipes. More specifically, a pipe joint is prepared from a shape memory alloy with an inner diameter at the pipe ends smaller than an outer diameter of pipes to be connected; and this shape of the joint is memorized; then at a low temperature the inner diameter of the joint is deformed to be larger than the outer diameter of the pipes to be connected; in this state the pipes are inserted into the joint with slight clearance between the pipes and joint; and then the memorized shape of the joint with the smaller diameter prior to the deformation is recovered by heating the connected areas to a proper temperature to fasten the pipes. In this case, since the tight junction can be obtained only by heating of the connected areas, the connecting workability is excellent. Accordingly, if such a joint is generalized, it seems that it extremely contributes to the art.
As shape memory alloys, there are known not only some nonferrous metal alloys including the above-mentioned Ni-Ti alloys and Cu alloys but also some ferrous metal alloys such as Fe-Pd alloys, Fe-Ni alloys and Fe-Mn alloys. Among others, Ni-Ti alloys have been actually used in the manufacture of pipe joints, since they are excellent in shape memory effect and mechanical properties. However, Ni-Ti alloys are very expensive. Furthermore, in the case of a pipe joint of a Ni-Ti alloy, after it is finished so as to have pipe ends with an inner diameter smaller than an outer diameter of pipes to be connected, the ends of the joint must be expanded to a diameter larger than the outer diameter of the pipes to be connected at a low temperature and while keeping that low temperature the pipes must be inserted into the joint from both ends thereof. Thus, the connecting process with this joint requires the low temperature and, therefore this joint is disadvantageous because of low workability of the connecting process.
On the other hand, "YOSETU GIJUTU (welding technic), September 1988, pp. 78.about.84" teaches that Fe-Mn alloy can be used as a material for pipe joints.
However, ferrous metal shape memory alloys are generally disadvantageous in low corrosion resistance. JP A 61-201761 discloses examples of ferrous metal shape memory alloys whose corrosion is improved by adding Cr. However, the Cr content taught is too low, i.e. not more than 10.0%, to achieve corrosion resistance well comparable with that of stainless steels. Furthermore, JP A 63-216946 teaches to improve corrosion resistance of ferrous metal shape memory alloys by adding Cr. Again, however, the Cr content taught is 10% or less and it is not taught how to realize a desired level of shape memory characteristics with the ferrous metal shape memory alloys having Cr, which is a ferrite former, in excess of 10% incorporated therein.
Further, as to general stainless steels, "Scripta Metallurgica, 1977, vol. 5, pp. 663.about.667" reports that SUS304 steel exhibits shape memory effect, if it is deformed at -196.degree. C. and then heated to room temperature, however, its memory recovery is too small to put it to practical use.